I'm not well versed in luci config but from what I have learned the following should help.
As always, be sure to have a full backup and know your device recovery methods.
If you want to remove access to the "Firewall" management portion of luci altogether on recent versions of OpenWrt (v22 though 24 ?), that might be easy.
I just tested this and it worked:
Backup file /usr/share/luci/menu.d/luci-app-firewall.json
Remove file /usr/share/luci/menu.d/luci-app-firewall.json
Logout of luci
Log back in to luci and the "Firewall" menu item in the "Network" menu should be gone.
Clear your browser cache if needed.
To restore access to the firewall menu in luci, restore the file /usr/share/luci/menu.d/luci-app-firewall.json file and be sure file ownership and permissions are correct.
Logout and back in to luci.
That prevents casual use of the web api for changing the firewall settings. If you want a more robust solution preventing it, you should look at changing ACLs and or more significant modifications of luci.
For more ideas / info on this search the forum, wiki, and github.
These threads in the forum were insightful: Disable certain Luci menu items - #2 by stangri and Description of the JSON in menu.d and acl.d? - #4 by jow so might be a good starting point for deeper understanding.
Good Luck!